if your 6 speed is "ultra" spaced (5 cm c/c) then the 7 speed shifters will index correctly. Otherwise you'll have some issues. I don't think they made any 6s ultra cassettes, only 6s ultra freewheels. Most 6s cassettes are Uniglide, too, limiting your 7s cassette options.
Long cage RD: buy the Deore from Nashbar currently @ $17.99. Can't beat it.
If you're going to buy a new wheel, buy a 8/9/10s compatible freehub body and a 4.5mm spacer to place before your 7s cassette. In this case you'll be Hyperglide and open to all cog sizes.